1. Field of the Invention
The present invention relates to a water-based ink suitable for use in ink-jet recording in which an ink is ejected and flown as droplets from an orifice, and these ink droplets are applied to the surface of a recording medium, thereby conducting recording, and an ink-jet recording method and instruments using such an ink, and particularly to an ink-jet ink capable of preventing the occurrence of feathering and bleeding and forming images high in color strength, and an ink-jet recording method and instruments using such an ink.
2. Related Background Art
Water-based inks have heretofore been principally used as ink-jet recording inks from the viewpoint of safety, odor and the like. There have been known inks in which one or more of various water-soluble dyes or pigments are dissolved or dispersed in water or a mixed solvent of water and a water-soluble organic solvent, and a humectant, a dye-dissolving aid, a mildewproofing agent and/or the like are added thereto as needed. For the past few years, ink-jet recording has conspicuously spread because it has such advantages as: an ink can be ejected in a proportion of several thousand droplets per second to conduct high-speed recording with ease, noise is scarcely produced, multi-color recording can be performed with ease, high-resolution recording can be effected, and recording can be conducted on plain paper.
With the development of low-cost and high-performance personal computers and the standardization of GUI environment in recent years, even image recording by printers or the like has been required to achieve high coloring, high quality, high-fastness properties, high-resolution recording, high-speed recording and the like. In response to this requirement, various technical ideas that a coloring material component is left on the surface of paper as much as possible to increase the optical density of images to be formed, edges of printed dots are made sharp, and the occurrence of feathering, bleeding and the like is lessened are also being proposed in ink-jet recording.
As the first example thereof, Japanese Patent Application Laid-Open No. 58-13675 has disclosed a method of controlling the absorption and spreading of recording dots in and on paper by adding polyvinyl pyrrolidone to an ink. As the second example thereof, Japanese Patent Application Laid-Open No. 3-172362 has also disclosed a method of controlling the absorption of an ink and the spreading of dots by adding a specific microemulsion to the ink.
As the third example in which a sol-gel transition phenomenon is applied to an ink, Japanese Patent Application Laid-Open Nos. 62-181372 and 1-272623 each have described an ink which is in a gel state at room temperature, but turns to a sol state by heating. It is said that according to such an ink, the penetration of the ink into paper can be controlled because recording on the recording medium is conducted in the sol state, and the ink turns to a gel state by its cooling.
As the fourth example, Japanese Patent Application Laid-Open No. 6-49399 has recently disclosed an ink, to which a compound having reversibly and thermally gelling property is added, and which has good coloring ability and fixing ability, causes little feathering, provides prints excellent in shelf stability, and is also excellent in reliability, and an ink-jet recording method and instruments using such an ink. The technical background thereof is based on a phenomenon that when an aqueous solution of a specific water-soluble polymer is gradually heated, its water-solubility is lowered, and so the solution becomes cloudy (a temperature at which such a phenomenon occurs is called "clouding point").
Typical examples of the water-soluble polymer include N-isopropylacrylamide, polyvinyl methyl ether, polyethylene oxide and hydroxypropylcellulose. Since these polymers have a negative temperature coefficient as to solubility, they are in a state separated and deposited from the solution at a temperature not lower than the clouding point. In such a deposited state, hydrophobic microgel is formed, and the viscosity of the solution decreases. When recording on a recording medium is conducted in the deposited state, the viscosity of the solution returns to the initial viscosity, i.e., increases, owing to a temperature drop on the recording medium, and so the penetration of the ink can be prevented.
As the fifth example, M. Croucher et al. have indicated problems involved in the conventional homogeneous inks and moreover proposed, as a future ink for ink-jet, a heterogeneous ink making use of a latex. M. D. Croucher and M. L. Hair, "Design Criteria and Future Directions in Inkjet Ink Technology", Ind. Eng. Chem. Res., 28, 1712-1718 (1989)!.
U.S. Pat. No. 4,246,154 has disclosed an ink in which fine particles of a vinyl polymer are colored with a dye and anionically stabilized. U.S. Pat. No. 4,680,332 has also disclosed a heterogeneous ink in which a water-insoluble polymer containing an oil-soluble dye and combined with a nonionic stabilizer is dispersed in a liquid medium. Further, U.S. Pat. No. 5,100,471 has proposed a water-based ink comprising a solvent and colored particles composed of a polymer core and a silica shell to which a dye has been bonded by covalent bonding, and described this ink as having such features as a brighter color is developed on paper, it is stable to temperature change, and it provides images high in water fastness.
On the other hand, as the sixth example, Japanese Patent Application Laid-Open No. 3-240586 has proposed, as a nonaqueous ink, an ink in which colored particles coated with a resin that can swell with a dispersion medium are dispersed in kerosene or the like. In this proposal, it is said that the ink is effective in, particularly, prevention of image feathering and of clogging at an orifice for ejecting ink droplets.
However, the above-described first and second examples of the prior art involve a problem of the fixing ability of the ink in that since the penetration into paper is prevented, the ink does not penetrate into the paper, and hence remains on the paper for a long time. A problem that color mixing (bleeding) occurs between inks of different colors also arises.
The ink based on the sol-gel transition of the third example involves a problem that running behavior may arise due to changes in storage temperature of the resulting prints, and so staining by color mixing and transfer due to running of images occurs.
The inks containing the reversibly and thermally gelling compound of the fourth example are unfit for a recording method in which one pixel is recorded at high speed in several tens of milliseconds or shorter like ink-jet recording because it uses a water-soluble cellulose ether, and so its viscosity increase is slow. If the ink is used in ink-jet recording, such a compound must be used in a low concentration because the upper limit of viscosity upon ejection of the ink is as low as at most 20 mPa.s. It is hence difficult to sufficiently achieve the thickening effect.
On the other hand, among the fifth example group, the ink in which the coloring material has been anionically stabilized involves a problem that a pH region in which the coloring material is stably dispersed is narrow, and the selection range of the dye is hence limited. In addition, it has a disadvantage that printed dots thereof are small in spread on paper, and an optical density (OD) required is hence hard to achieve. With respect to the reduction of fixing time, which is required for high-speed recording, the ink has little effect because fixing depends on only evaporation and penetration like the conventional image-forming means.
According to the disperse ink comprising a polymer containing the oil-soluble dye and combined with the nonionic stabilizer, the selection range of the dye can be widened. As with the above ink, however, this ink has little effect on the shortening of fixing time because fixing is based on a mechanism depending on evaporation and penetration. In addition, the ink is disadvantageous even from the viewpoint of color mixing (bleeding) between inks of different colors because it takes a lot of time to fix adjacent dots.
The disperse ink of the polymer core/silica shell structure is excellent in dispersion stability of the pigment, but does not provide a sufficient OD because the ink has no particular means for the aggregation of the coloring material on the surface of paper. In addition, the ink has little effect on the reduction of fixing time because fixing depends on evaporation and penetration. Therefore, the ink involves a problem that bleeding occurs.
A problem common to the above three examples, is that the rub-off resistance of recorded images is poor because the adhesion of the coloring material to the surface of paper is not taken into consideration in such inks.
The sixth example in the prior art involves problems of odor, safety and the like because it uses kerosene as a dispersion medium.
To eject ink droplets, water-based inks, particularly, ink-jet inks are required to have such physical properties as the surface tension is greater than 20 dyn/cm (related to refilling speed), the viscosity is within a range of from 1 to 20 mPa.s, pH is about 3 to 10, and fixing time is shorter than 20 seconds (it is better to be the shortest possible time).
Here, the transfer of ink to paper is considered. With respect to the transfer phenomenon of liquid to paper, the Lucas-Washburn equation has been generally known. Supposing an amount of the liquid transferred, a roughness index of the paper, an absorption coefficient, transferring time and wetting-starting time are V, Vr, Ka, T and Tw, respectively, in case the liquid is water, the amount of the liquid transferred V is represented by the equation ##EQU1##
In the equation (1), Ka is related to physical properties of both paper and ink and is represented by the equation ##EQU2## wherein r is a capillary radius of the paper, .gamma. is a surface tension of the liquid, .theta. is a contact angle, and .eta. is a viscosity of the liquid.
It is understood from the equation (1) that in order to leave a coloring material on the surface of the paper, it is necessary to delay the penetration of the liquid, namely, make Ka small, as far as possible (the time of evaporation can be gained by making Ka small). It is understood from the equation (2) that it is only necessary that the surface tension .gamma., viscosity .eta. and contact angle .theta. be made low, high and great, respectively, as physical properties of an ink required to do so. However, the physical properties of the ink-jet ink as described previously are variously limited. It is thus difficult to control Ka.
On the other hand, when a nonaqueous solvent, for example, ethanol, is used as the liquid, the wetting-starting time Tw in the equation (1) may be ignored. Therefore, fixing can be accelerated. However, Ka also becomes greater, and so the penetrating effect is increased, resulting in an image on which "feathering" occurs to a great extent. In addition, the term of cos .theta. in the equation (2) is determined by the combination of ink and paper. Therefore, whether the quality of images is good or poor depends on the kind of paper used. Namely, such an ink cannot satisfy paper nondependence.
It is considered that the above-mentioned problems may arise even on the conventional coloring material-dispersed inks so far as their image formation depends on penetration and evaporation.